Remove django-redis hard dependency

[frague59]

Hi,

Thanks for this great project !

As django-redis has become an optional dependency to use django redis backend, it should be nice to remove this dependency to the django-prometheus project.

From django 4.2, django provides the "django.core.cache.baskends.RedisCache", using the more generic redis[hiredis] wrapper to redis connections, and not django-redis.

Maybe we should wrap the django_redis import in try / except about ImportError ? This should allow to remove hard dependency to django-redis.

Thanks !